- A Should always be on, in any condition
- B Help slow a vehicle, reducing brake wear Correct answer
- C Replace the foot brake entirely
- D Are only for downhill use
Why this is the correct answer
Retarders (engine, exhaust, hydraulic, or electric) reduce brake wear by absorbing energy. They should be turned off on slick roads to prevent skidding.
How this topic appears on the test
The General Knowledge CDL exam covers Shifting gears as a recurring theme. Test-writers favor questions that probe whether the applicant has internalized the safe operating procedure, not merely memorized the rule. When you encounter a question on this topic, slow down and verify that the answer you've chosen describes what a careful, defensive driver would actually do — not what is technically permissible.
If you missed this question, that's exactly the right outcome of a practice session: the goal is to surface gaps before the actual test. Open your state CDL manual to the Shifting gears section and re-read the relevant subchapter. Then come back and run through the rest of the General Knowledge practice exam.
For drivers preparing for the General Knowledge CDL exam, additional context — including federal manual excerpts, employer hiring practices, and DOT medical guidance — is widely available from industry resources. Continue reading on a recommended industry resource for further detail. Always cross-check anything you read with the current edition of your state CDL manual, since enforcement guidance is updated periodically.